Radio frequency microelectromechanical systems (MEMS) switches are of paramount in importance for future miniaturizations of radio frequency systems. Space-based radar, phased array radar and phase shifters all depend on being able to easily and reliably switch between different radio frequency loads. Because of their small geometries, exceptional performance and low power consumptions, MEMS metal contact switches are well suited for these applications.
Important performance criteria for many MEMS metal contact switch applications are low contact resistance, resistance preferably less than one ohm and not more than two ohms in magnitude, and high reliability; for example an operating life of more than 108 ‘hot-switched’ cycles. The two primary failure mechanisms for metal contact switches involve the switch becoming stuck in the closed position (i.e. stiction) or incurring increasing contact resistance as a result of increasing switch operating cycles. Typically, MEMS metal contact switches use gold-on-gold electric contacts in order to achieve the desired low contact resistance. This low resistance results of course from gold's low electrical resistivity and its resistance to surface oxide and sulfide layer formations. However, MEMS switches with gold electric contacts are prone to the stiction and increased contact resistance failure mechanisms because of gold's relatively low hardness, a hardness of 1-2 GPa is typical.
Previous work in this field has focused on the optimizing of mechanical switch configurations rather than investigating different electric contact metallurgies. Notable exceptions to this focusing on the optimizing of mechanical switch configurations are the work of S. Majumder et al. and S. Duffy et al. which involve ‘platinum group’ and platinum electric contact metals, respectively [1, 2]. Bracketed numbers such as this refer to individual items in the list of references appearing in the APPENDIX located at the end of this specification; these references are hereby incorporated by reference herein. These metals were chosen over gold for their increased hardness and improved wear characteristics. In order to achieve acceptable contact resistance values, the Majumder et al. switches require multiple, parallel contacts and are packaged in a novel hermetic environment while the Duffy et al. switches require very high actuation voltages of about 80 Volts.
J. Schimkat studied gold-nickel alloy (Au-(5 at %)Ni) macro-switch contacts in a low-force test configuration but did not fabricate or test actual MEMS devices [3]. Currently, there are no other known works appearing in the open published literature describing how to select and incorporate metal alloys as micro-switch electric contact materials. The present invention provides a method for selecting metal alloy electric contact materials for micro-switches that are optimized for increased wear, low contact resistance and low susceptibility to oxidation, to contaminant gettering and to the formation of sulfide layers. The present invention is thus believed to provide a desirable contribution toward resolution of the MEMS radio frequency switch contact metal difficulty.